An optical coupling device that can transmit a signal while its input terminal and output terminal are electrically insulated from each other, has become popular in electronic equipment that employs different types of power source systems, such as a DC power source system, an AC power source system, and a telephone line system. In such optical coupling devices, a transistor output photocoupler, in which a phototransistor is employed as a light receiving element, has wide versatility and wide applicability.
The optical coupling device is characterized by a current transmission rate (CTR), which indicates a current flowing into a light receiving element Ic as a percentage of a current flowing into a light emitting element IF. The current transmission rate of the optical coupling devices is required to have a wide range so that it can be used in different applications. Further, a switching time of the optical coupling devices is required to be small regardless of changes in the load resistance.